This invention relates to systems for sealing the instrumentation ports associated with nuclear reactor systems. More particularly, the present invention relates to high quality clamps and systems for maintaining a proper seal at the interface between reactor vessel head penetrations and the thermocouple instrument columns.
Due to the risks associated with operating a nuclear power plant, the design and quality standards associated with nuclear reactor equipment are extremely high and stringent. Accordingly, problems which are capable of straight forward solution in a non nuclear environment are difficult and demanding in the context of a nuclear reactor facility. For example, it is generally required in many industrial settings to monitor the pressure, temperature, and other parameters of various operating equipment. In the environment of a nuclear power plant, leaks are extremely dangerous due to the high potential for escape of radioactive materials into the containment building. Accordingly, it is imperative in these situations that the instrumentation used to make such measurements be precisely designed to prevent such leaks.
While the possible escape of radioactive material from a nuclear power plant is minimized by the containment building surrounding the nuclear steam supply system, the working conditions inside the containment building are hazardous at all times after the fuel is activated. This is even true during refueling of the reactor high airborne particulate radioactivity exisss in the containment building. Safety regulations set maximum radiation dose limits for workers in these locations during plant operation and refueling. Many of these same locations also impose a difficult working environment during refueling due to the high ambient temperatures inside the containment building. In many situations, these locations are also not easily accessible and a safe work platform is not available. It is accordingly desirable to provide clamping system which are quickly and easily assembled so as to minimize worker exposure to such hazardous conditions. Such quick and efficient repair and/or replacement of instrument port clamps and clamping systems is also highly desirable from the economic point of view since it minimizes the down time of the nuclear plant and hence the cost of providing replacement electricity. In particular, the instrument port assemblies must be disassembled before the reactor vessel head can be removed and then reassembled after refueling is complete and the vessel head is installed.
In order to more clearly understand the present invention, a typical heretofore used instrumentation port interface assembly is revealed in FIG. 1. As revealed by this illustration, a lower conduit or flange 10 is coupled or otherwise mounted to a vessel while the upper conduit or flange 11 is coupled or otherwise mounted to flange 10 and assembly 42. In the particular application of a nuclear power plant, flange 10 is the female flange having its lower end threaded and welded onto the vessel head penetration. The flange 11 is the male flange, and assembly 42 is the conduit seal which houses the thermocouples which pass into the interior of the male flange and down into the reactor internals. Flanges 10 and 11 are generally tubular in shape and have upper and lower surfaces respectively which are designed to engage one another in a sealing manner with respect to gasket 12. The conduit seal 42 is also generally tubular and cooperates with the male flange 11 in a telescoping manner to seal the interface therebetween. A gasket 12A engages both the male flange and the conduit seal 42 at the interface therebetween. In order to effectively compress gasket 12 and seal the interface between flanges 10 and 11, it is necessary to exert axial pressure on flanges 10 and 11 such that pressure exerted on each flange is directed towards the interfacing end of that flange. That is, a clamping apparatus should exert an upward axial force on flange 10 while exerting a substantially equal and opposite downward force on flange 11. In this way, the interface between flanges 10 and 11 is properly sealed by gasket 12. In contrast, the interface between male flange 11 and assembly 42 requires application of axial pressure to each flange which is directed away from the interfacing ends thereof. That is, it is necessary for a clamping apparatus to exert an upward axial pressure on conduit seal 42 with respect to the male flange 11. In this way, the interface between the male flange 11 and the conduit seal 42 is properly sealed by gasket 12A.
The seal between flanges 10, 11, and 42 is an important safety consideration in the design of nuclear steam supply systems. It will be appreciated by those skilled in the art that such flange interfaces are generally located in regions of the plant having a high radioactivity level and high process temperatures. Because of these special circumstances, high quality clamps capable of sealing the interface between flanges are not only desirable but necessary. In some applications, it is desirable to construct such clamps from high strength material. In addition, it is highly critical to worker safety that the clamping apparatus used to seal such interfaces be quickly and easily installed and removed.
One clamping apparatus generally designated as 20, which has heretofore been used to seal the interface between the female flange 10 and the male flange 11 is shown in FIG. 2. The clamping apparatus 20 consists of three essentially identical body members 13A, 13B, and 13C. Each body member spans an arc of approximately 110.degree.. An interbody gap 15 of about 10.degree. exists between the body members. Each end of the body members 13A, 13B and 13C contains a flanged portion which is used to attach the body members together. A cap screw 14 (as shown) or other holding means is passed through the flanged ends and holds the body members in a generally ring-shape while the clamp is assembled on flanges 10 and 11.
The use of the existing clamp 20 on the interface between the male flange and female flange as shown in FIG. 1 will now be described. Due to its configuration and weight, the clamp 20 of FIG. 2 is generally brought to the instrument port in disassembled form. At least two workers are then generally required to assembly clamp 20 in situ around the outer portion of the interface between flanges 10 and 11. Workmen only have access to flanges 10 and 11 from radially outside the reactor vessel head because of the cooling shroud and other equipment permanently installed thereabove. The specified procedure for operation of the heretofore used clamping apparatus requires the use of an axial loading device which seats the gasket prior to the application of the clamp. Such axial loading devices are generally cumbersome, heavy and tend to interfere with the vessel head shroud, making the installation thereof extremely difficult. The application of this axial loading device also restricts the work space available and therefore complicates the assembly of clamp 20. Once the axial loading device is properly positioned, the interbody gaps 15 must be carefully adjusted so as to be substantially equivalent in order to achieve generally uniform contact and pressure on the flanges 10 and 11, and to minimize cap screw shank bending. The cap screws 14 are generally torqued to about 100 ft-lb or less. It should be noted that, in many applications, over torquing of the cap screws 14 may result in overcompression of gasket 12 when certain gasket configurations are used. For many gaskets, overcompression has a serious detrimental impact on the sealing capacity of the gasket. Some prior art clamping apparatus generally used space limiters between the flanges in order to prevent such overcompression of the gasket. It is apparent from the above description that the procedures and apparatus required for the assembly of clamp 20 and other prior clamping devices are thus time consuming and present a large potential for improper installation. The above disadvantages are even more pronounced when it is considered that such a clamp must be installed in awkward and precarious positions requiring workers to be tethered by ropes and/or other safety gear and that workers are required to wear cumbersome gear such as masks, heavy gloves, and radiation suits with respirators.
The apparatus which have generally been heretofore used to seal male flange 11 and the conduit seal 42 incorporates the use of six separate jack screws. Each jack screw must be separately torqued in stages to provide the necessary axial clamping force around the entire perimeter of the assembly 42 and male flange 11. The jack screws are disposed circumferentially around the perimeter of the conduit seal 42 and male flange 11 through a ring called a jack screw plate. The jack screw plate is in contact with a protruding portion of a split ring which is disposed within annular recess 43 formed in the conduit seal 42. The lower end of each jack screw is in contact with shoulder IIA which is formed in the top of male flange 11. The threads of each jack screw cooperate with threads formed in circumferentially spaced holes formed in the jack screw plate. Axial loads developed by torquing the jacks are transmitted to the groove in the conduit seal 42 by way of the splitting. To provide the necessary sealing force between the conduit seal 42 and the male flange 11, the jack screws are torqued, thereby causing the conduit seal to lift relative to the male flange. Thus, the base of the jack screws apply a downward force to the male flange 11 while the jack screw plate applies an equal and oppositely directed force to the split ring, which, in turn, is transmitted to the conduit seal 42.
Several disadvantages are attendant with the use of the jack screw system described above. For example, each jack screw may be turned, individually, only a few turns before adjacent jack screws must then be torqued in order to minimize cocking of the conduit seal. Therefore, the remaining five jack screws must each, individually, be turned a small number of turns to move the jack screw plate evenly. The process must be repeated several times until the jack screws and the jack screw plate apply the necessary uniform force between the conduit seal 42 and the male flange 11. This a cumbersome and time consuming procedure, especially in the hazardous environment of nuclear power plants. Moreover, over torquing or improper advancement of the jack screws may cause one or more of the jack screws to be bent. This in turn necessitates replacement of the jack screws and, if the threads of the jack screw plate have also been damaged, then the entire jack screw plate may require replacement. Another potential disadvantage of the jack screw system is the criticality of the torquing requirements on each jack screw. That is, if the jack screws are not torqued evenly, an unequal sealing force will occur around the port column and male flange perimeter which may possibly compromise the effectiveness of the seal therebetween. Furthermore, after the jack screw assembly is completed, lock wire must be properly supplied to prevent the jack screws from becoming loose during plant operation.